The ancient Romans are thought to have been the first to recover metal using a biological process. It is believed that they took advantage of natural sites to recover copper sulfate resulting from the microbial biooxidative acid leaching of sulfide ore. Details of the copper recovery process were not documented until 1670 at Rio Tinto in Spain. The methods documented in 1670 are still used today. One organism instrumental in the modern use of this biological recovery process is Thiobacillus ferrooxidans, which was isolated in 1947 from an abandoned coal mine in West Virginia (Colmer [1947]). This organism's activity is normally limited by the amount of substrate exposed to the atmosphere where the necessary oxygen is present to carry out the oxidation of the metal sulfide.
Although deposits of high grade ore are frequently most efficiently extracted by smelting, it has been found that low grade inherent deposits of sulfide ore can be extracted by heap leaching which utilizes the biooxidation processes (Manchee [1979]). In this process, roughly fist-sized lumps of ore are piled upon a surface that is impervious to liquids. Water is applied to the top of the heap by a sprinkler system to provide moisture for the natural flora and to create a constant flow of fluid throughout the pile. Within the heap, two methods of leaching occur. The first occurs at the surface of the pile where the natural flora, in the presence of oxygen, attack iron pyrites producing ferric sulfate and sulfuric acid. The bacteria likewise act upon other mineral sulfides to dissolve other metallic minerals as well. In the center of the heap, out of the presence of oxygen, a second leaching process occurs. This indirect leaching is a purely chemical process by which the highly oxidizing acidic ferric sulfate produced by the bacterial leaching process at the surface of the pile reacts with mineral sulfides and oxides. Effluent running from the heap containing the products of these two leaching processes can be collected and treated to recover purified metal ore.
Normally the heap need not be inoculated with extraneous bacteria but rather the heap's natural flora is relied upon for the leaching process. Similar biooxidative processes are exploited for in situ leaching situations. Enargrite, Cu.sub.3 (As,Sb)S.sub.4 ; Chalcopyrite, CuFeS.sub.2 ; Bornite, Cu.sub.5 FeS.sub.4 ; Covellite, CuS; Chalcocite, Cu.sub.2 S; Tetrahedrite, Cu.sub.8 Sb.sub.2 S.sub.7 ; and Chalcomenite, CuSeO.sub.3.2H.sub.2 O are examples of the copper ores which are amenable to this biooxidative process. Other metals that can be recovered by the process of sulfide biooxidation include iron, molybdenum, nickel, lead, arsenic, antimony, tin, uranium, vanadium, gold, and zinc.
In the case of copper recovery, copper sulfide breaks down more slowly than copper oxide because sulfide minerals are composed of reduced sulfur and iron that must be oxidized in order for dissolution to occur. Bacterial oxidation is important to the dissolution of copper sulfide minerals, particularly chalcopyrite, and to a lesser extent, chalcocite.
Several kinds of bacteria have been described that oxidize reduced forms of iron and/or sulfur including Thiobacillus ferrooxidans, T. thiooxidans, Leptospirillum ferrooxidans, Sulfolobus, Acidianus, Sulfobacillus, Strain ALV, Strain LM2, and others. New strains of mineral-oxidizing bacteria have more recently been described by Huber and Stetter (1989, 1990) called T. cuprinus and T. prosperus.
While T. ferrooxidans has received the most attention, as mentioned above, many other microbes are present in heaps and dumps that can contribute to the extraction of copper. Most of these sulfide/iron-oxidizing bacteria are capable of using carbon dioxide to satisfy their carbon needs. Many organic compounds inhibit T. ferrooxidans (Torma et al. [1976]; Tuttle and Dugan [1976]; and Puhakka and Tuovinan [1987]). T. cuprinus, however can use certain forms of organic carbon. The limiting nutrients for most bacteria found in the ore containing environment are nitrogen and phosphorus. The most preferred form of nitrogen for these organisms is ammonium; phosphate serves as a source of phosphorus. These nutrients must be solubilized before the bacteria can use them. Phosphate requirements may also be satisfied if minerals such as apatite are present and solubilized. Some strains of T. ferrooxidans are able to use atmospheric nitrogen if ammonium nitrogen is not available (Stevens et al. [1986]).